Exhaust gas purification apparatus for diesel engine

ABSTRACT

A DPF disposed in an exhaust passage of a diesel engine is coated with a selective reducing catalyst, for selectively reducing and purifying NOx by using a reducing agent, and a thin film, having fine pores of size for allowing passage of the NOx and preventing passage of PM, in this order. In regeneration treatment of a DPF, the thin film prevents direct transmission of combustion heat of the PM to the selective reducing catalyst and suppresses a temperature rise of the selective reducing catalyst to thereby suppress deterioration of an active component.

This application is a continuation of PCT/JP2009/065521, filed on Sep.4, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for removing nitrogenoxides (NOx) and particulate matter (PM) in exhaust gas in an exhaustgas purification apparatus for a diesel engine.

2. Description of Related Art

As an exhaust gas purification apparatus for simultaneously removing NOxand PM included in exhaust gas of a diesel engine, an apparatusincluding a diesel particulate filter (DPF) coated with a NOx selectivereducing catalyst as disclosed in Japanese Laid-Open (Kokai) PatentApplication Publication No. 2006-274986 has been proposed. In thisexhaust gas purification apparatus, the PM is trapped and removed by theDPF while the NOx is selectively reduced and purified in the NOxselective reducing catalyst by using ammonia, produced from a ureaaqueous solution injection-supplied on an exhaust upstream side of theDPF, as a reducing agent.

In the DPF, if clogging develops as an amount of trapped PM increases,exhaust pressure increases over an acceptable value to result in lowfuel economy. Therefore, regeneration treatment is indispensable, in theregeneration treatment, the PM trapped by the DPF is incinerated by thewell-known method such as increasing exhaust temperature by an enginecontrol and actuating an electric heater provided to the DPF. Becausetemperature of the DPF increases due to incineration heat of the PMduring the regeneration treatment of the DPF, temperature of the NOxselective reducing catalyst applied to the DPF increases as well. In theNOx selective reducing catalyst, there is a possibility that an activecomponent is deteriorated by heat due to the excessive temperature rise.

SUMMARY OF THE INVENTION

Therefore, in view of the above conventional problems, the presenttechnique has an object to provide an exhaust gas purification apparatusfor a diesel engine, in which it is possible to prevent directtransmission of incineration heat of the PM to the NOx selectivereducing catalyst applied to the DPF to thereby suppress deteriorationof an active component.

In the exhaust gas purification apparatus, a DPF disposed in an exhaustpassage of a diesel engine is coated with a selective reducing catalyst,for selectively reducing and purifying NOx by using a reducing agent,and a thin film, having fine pores of size for allowing passage of theNOx and preventing passage of PM, in this order.

With this exhaust gas purification apparatus, the PM included in exhaustgas of the diesel engine cannot pass through the thin film and thereforeis trapped in a surface of the thin film. On the other hand, the NOxincluded in the exhaust gas passes through the thin film, reaches theselective reducing catalyst, and is selectively reduced and purified bythe selective reducing catalyst by using a reducing agent. Duringregeneration treatment of the DPF, the thin film prevents directtransmission of incineration heat of the PM to the selective reducingcatalyst to suppress a temperature rise of the selective reducingcatalyst. As a result, it is possible to suppress deterioration of anactive component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block view illustrating an example of an exhaustgas purification apparatus in which the present technique is embodied;

FIG. 2 is an explanatory view of a state in which a DPF is coated with aNOx selective reducing catalyst and a thin film; and

FIG. 3 is an enlarged view of a portion A in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereunder is a detailed description of the present technique withreference to the accompanying drawings.

FIG. 1 is a general block view of an exhaust gas purification apparatusfor purifying NOx and PM included in exhaust gas of a diesel engine byusing a urea aqueous solution which is a reducing agent precursor.

In an exhaust pipe 14 (exhaust passage) connected to an exhaust manifold12 of a diesel engine 10, a DPF16 for trapping and removing the PM inthe exhaust gas is disposed. The DPF 16 has a large number of cellsformed substantially parallel to an exhaust gas flow by partition wallsformed of porous members such as ceramics and inlets and outlets of therespective cells are alternately sealed with sealing members in astaggered pattern. When the exhaust gas in the cell with the sealedoutlet flows into the adjacent cell with the sealed inlet through thepartition wall, the PM in the exhaust gas is trapped by the porousmember forming the partition wall.

As illustrated in FIG. 2, an exhaust inflow face of the DPF 16 is coatedwith a NOx selective reducing catalyst 18, for selectively reducing andpurifying the NOx by using ammonia as a reducing agent, and a thin film20, having fine pores of size for allowing passage of the NOx andpreventing passage of the PM, in this order. Here, as a method ofapplying the NOx selective reducing catalyst 18, for example, an activecomponent may be directly supported on a surface of the DPF 16 or awashcoat of a carrier having an active-component-impregnated layer maybe applied on the surface of the DPF 16. On the other hand, as the thinfilm 20, it is preferable to employ alumina or zeolite in order to makeit easy to form the thin film 20.

A spray nozzle 22 for spraying the urea aqueous solution is disposed inthe exhaust pipe 14 on an exhaust upstream side of the DPF 16. The ureaaqueous solution stored in a reducing agent tank 24 is supplied to thespray nozzle 22 via a reducing agent dosing device 26 including a pumpand a flow rate control valve.

As a control system of the exhaust gas purification apparatus, in theexhaust pipe 14 on the exhaust upstream side of the spray nozzle 22, atemperature sensor 28 for measuring temperature (exhaust temperature) ofthe exhaust gas introduced into the DPF 16 is disposed. An output signalof the temperature sensor 28 is input to a reducing agent dosing controlunit (hereafter referred to as “DCU (Dosing Control Unit)”) 30 includinga computer. The DCU 30 is connected to an engine control unit (ECU) 32so as to be able to communicate with the ECU 32 through a network suchas a CAN (Controller Area Network) in order to appropriately read in anengine rotation speed and an engine load as engine operating states. Asthe engine load, a known state quantity such as a fuel injection amount,torque, an accelerator opening, a throttle opening, an intake flow rate,intake negative pressure, and supercharging pressure may be used.

The DCU 30 executes a control program stored in ROM (Read Only Memory)to compute the urea aqueous solution dosing flow rate suitable to theexhaust temperature, the engine rotation speed, and the engine load, tothereby output a control signal according to the dosing flow rate to thereducing agent dosing device 26. In the reducing agent dosing device 26,the pump and the flow rate control valve are appropriately andelectronically controlled based on the control signal from the DCU 30,and the urea aqueous solution of the flow rate according to the engineoperating state is supplied to the spray nozzle 22.

In this exhaust gas purification apparatus, the urea aqueous solutionsprayed from the spray nozzle 22 into the exhaust pipe 14 is hydrolyzedby exhaust heat and water vapor in the exhaust gas and converted intoammonia and the ammonia flows in the exhaust flow and is supplied to theDPF 16. The thin film 20 applied on the exhaust inflow face of the DPF16 has fine pores of size for allowing passage of the NOx and preventingpassage of PM, and thus, as illustrated in FIG. 3, the PM 34 in theexhaust gas is trapped in the surface of the thin film 20 and the NOx 36in the exhaust gas passes through the thin film 20 and reaches the NOxselective reducing catalyst 18. The NOx selective reducing catalyst 18selectively reduces and purifies the NOx 36 in the exhaust gas by usingthe ammonia supplied while flowing in the exhaust flow.

Then, for example, even if the exhaust temperature is increased by anengine control to ignite the PM 34 in order to carry out regenerationtreatment of the DPF 16, existence of the thin film 20 prevents directtransmission of incineration heat of the PM 34 to the NOx selectivereducing catalyst 18. As a result, it is possible to suppress atemperature rise of the NOx selective reducing catalyst 18 to therebysuppress deterioration of the active component.

At this time, because the NOx selective reducing catalyst 18 and thethin film 20 are applied only on the exhaust inflow face of the DPF 16,exhaust-passing resistance of the DPF 16 coated with the NOx selectivereducing catalyst 18 and the thin film 20 can be reduced and it ispossible to suppress reduction in fuel economy and output due toincrease in exhaust pressure. The thin film 20 is made of alumina orzeolite, and thus, it is easy to form the thin film 20.

Although the exhaust gas purification apparatus described above is basedon the premise that the urea aqueous solution is used as the reducingagent precursor, it is also possible to use hydrocarbon, alcohol, dieseloil, solid urea, and the like as the reducing agent or the precursorthereof according to a mechanism for selectively reducing and purifyingthe NOx. The technique can be applied not only to the device in whichthe reducing agent or the precursor thereof is injection supplied intothe exhaust pipe 14 but also to a device in which a catalytic converteris used to produce a reducing agent from exhaust components.Furthermore, the NOx selective reducing catalyst 18 and the thin film 20may be applied on an exhaust oufflow face as well as on the exhaustinflow face of the DPF 16. Here, because the thin film 20 applied on theexhaust oufflow face of the DPF 16 does not contribute to trapping ofthe PM, it is preferable to apply only the NOx selective reducingcatalyst 18 on the exhaust outflow face of the DPF 16 from a viewpointof reduction of the exhaust-passing resistance.

It should be noted that the entire contents of Japanese PatentApplication No. 2008-230820, filed on Sep. 9, 2008, on which theconvention priority is claimed is incorporated herein by reference.

It should also be understood that many modifications and variations ofthe described embodiments of the technique will occur to a person havingan ordinary skill in the art without departing from the spirit and scopeof the present technique as claimed in the appended claims.

1. An exhaust gas purification apparatus for a diesel engine,comprising: a diesel particulate filter disposed in an exhaust passageof the diesel engine; a selective reducing catalyst for selectivelyreducing and purifying nitrogen oxides by using a reducing agent; and athin film, having fine pores of size for allowing passage of thenitrogen oxides and preventing passage of particulate matter, whereinthe diesel particulate filter is coated with the selective reducingcatalyst and the thin film in this order.
 2. An exhaust gas purificationapparatus for a diesel engine according to claim 1, wherein the thinfilm is applied only on an exhaust inflow face of the diesel particulatefilter.
 3. An exhaust gas purification apparatus for a diesel engineaccording to claim 1, wherein the thin film is made of alumina orzeolite.